When a business fails or a debtor stops paying, the outcome is decided almost entirely by paperwork filed years earlier. This article explains how security interests and liens actually work: how a creditor acquires rights in someone else's property, what makes those rights good against the rest of the world, and who gets paid in what order. It covers attachment and perfection under Article 9, the financing statement that does the work, and the purchase money priority that jumps the queue. It explains repossession and disposition, including the self-help rules that create liability when they go wrong. It covers mechanics liens, tax liens, and judgment liens, and how bankruptcy reorders everything. It closes with practical guidance for lenders, sellers, contractors, and debtors.